Most containers made of foamed polystyrene materials are currently manufactured by steam chest molding techniques wherein expandable polystyrene beads are introduced into a closed mold cavity and expanded by means of hot steam introduced into the mold cavity. A foamed polystyrene container results having the contours of the cavity. While this conventional foamed container manufacturing method has been successfully used for containers of all sizes, it does have drawbacks in that the molds and equipments are expensive and the cycle time of the molds is dependent upon the time required to expand the beads in the mold cavity and the mold cool down time.
There have been several attempts to develop methods and apparatus for continuously forming foamed plastic containers by using apparatus other than the aforementioned steam chest molding apparatus. These include, the methods shown in U.S. Pat. No. 4,028,034 to Hahn and U.S. Pat. No. 3,225,127 to Scott. Both of these prior art patents disclose extrusion of an expandable foam parison that is subsequently blow molded to form containers. However, the patent to Hahn relies on a quadrasectional mold that is not well adapted to continuous processing. The patent to Scott addresses only the formation of bottles of the type having a small neck (as opposed to a wide mouth) with the neck being molded to form a high tolerance neck finish simultaneously with the blowing of the parison.
According to applicants invention, there is provided a method and apparatus for continuously forming thin walled open mouthed foamed plastic containers by blow molding techniques that provide both speed and reliability.
The basic technique of the invention includes introducing a foamable plastic material to an extruder to produce a tubular parison of the material that is subsequently clamped between mold halves moving both axially and radially inwardly to form cavities for open mouthed containers. According to a preferred form of the invention, cavities for two containers are provided in each set of mold halves which are clamped together around the parison at the middle of the mold set to provide individual container cavities extending on either side of the clamped portion, said individual cavities terminating in the mouth portion of the container cavity which is joined in a mouth to mouth relationship with an individual container cavity of the next set of mold halves. A blowing gas is inserted into the thus clamped parison to expand it into the mold cavities to assume the shape thereof and to form on a continuous basis sets of two containers joined together in mouth to mouth relationship with containers formed in the cavities of the adjacent mold sets.
According to a feature of the invention, the containers thus formed are severed while still in the mold by passing a cutting and severing means through the plastic material that joins said containers in mouth to mouth relationship. The mold halves are then moved radially apart to permit removal of the containers thus formed.
According to another feature of the invention, there is provided a method and apparatus for extruding a parison, wherein a portion of the molten plastic material is removed from the extruder at a point upstream from the point of introduction of a foaming agent for the plastic material. The portion removed is then routed to the extrusion head where it is coextruded with the material containing foaming agent thus resulting in a coextruded parison having an outer layer of plastic material that contains no foaming agent and an inner layer of the same plastic material that contains a foaming agent. The resulting product is one where the outer layer provides a smooth and glossy surface finish of unfoamed plastic material to the container comprised mainly of foamed plastic material.
It was an object of the invention to provide a continuous blow molding method wherein foamed plastic open mouthed plastic containers could be formed.
It was another object of applicants invention to provide a method of forming foamed open mouthed plastic containers on an end to end basis and to provide a means for severing such containers to provide a clean cut on the open mouth portions thereof.
It was a further object of the invention to provide a smooth and glossy surface finish on such containers.
These and other objects of the invention will be apparent from the drawings and the following description thereof.